Increase overall body weight through a calorie surplus
Build core and oblique muscles with resistance training
Do weighted side bends
Do cable woodchops
Do Russian twists with added weight
Do farmer’s carries with heavy loads
Do barbell squats and deadlifts
Train the lower back and hips
Reduce excessive cardio if trying to gain size
Eat more protein, carbohydrates, and healthy fats
Increase meal frequency if needed to reach calorie goals
Track body weight and waist measurements regularly
Consult a doctor or trainer before making major training or diet changes